1. Field of the Invention
The present invention relates to an image pickup optical system and an image pickup apparatus having the same, suitable, for example, for a video camera, a digital still camera, a TV camera, and a surveillance camera, each of which uses an image sensor, and a film-based camera that uses a silver film.
2. Description of the Related Art
A so-called telephoto type image pickup optical system (telephoto lens) is known as an image pickup optical system having a long focal length, and includes, in order from an object side to an image side, a front lens unit having a positive refractive power and a rear lens unit having a negative refractive power. The “long focal length,” as used herein, means, for example, a focal length longer than the size of an effective image pickup range. In general, as the focal length becomes longer in a telephoto lens having a long focal length, a large amount of a chromatic aberration, such as a longitudinal chromatic aberration and a lateral chromatic aberration, particularly occurs among a variety of aberrations.
JP H09-145996 (corresponding to U.S. Pat. No. 5,757,555) discloses an (achromatic) telephoto lens in which a positive lens made of a low dispersion material that has an abnormal partial dispersion characteristic, such as fluorite or brand name S-FPL51 (Ohara Inc.) is combined with a negative lens made of a high dispersion material so as to correct these chromatic aberrations. This reference discloses a telephoto lens having a focal length of 294 mm to 392 mm and an F-number of about 4.08 to 5.6. In addition, as the F-number becomes smaller, large amounts of a spherical aberration and a coma particularly occur among a variety of aberrations in the telephoto lens having the small F-number.
In order to correct the spherical aberration and the coma in the telephoto lens having the small F-number, JP H08-327897 (corresponding to U.S. Pat. No. 5,745,306) discloses a telephoto lens that has an increased number of lenses and an increased degree of freedom to the aberrational corrections. This reference discloses a telephoto lens having a large aperture ratio, an F-number of about 2.88 to 4.08 and a focal length of 294 mm to 588 mm, and increases the lens number so as to correct the aberration.
One known method uses a refractive optical element in which a refractive optical unit configured to provide diffractions is provided on a substrate, for a lens surface or part of an optical system, so as to correct the variety of aberrations containing a chromatic aberration of the optical system and to reduce the weight of the lens. JP 2009-271354 discloses an optical system that reduces an overall lens weight by using this method to correct the chromatic aberration, to shorten the overall lens length (distance from the first lens surface to the image plane), and to make a lens of a glass material having a comparatively small specific gravity.
This reference discloses a telephoto lens having a large aperture ratio, a focal length of 293 mm to 391 mm, and an F-number of about 2.9 to 4.1, thereby excellently correcting a chromatic aberration using a diffractive optical element. This reference shortens an overall lens length, corrects a monochrome aberration of a single color using an aspheric surface by an increased power of the first lens unit, and rectifies the chromatic aberration using a diffractive optical element. This configuration corrects various aberrations, and reduces an overall size and an overall weight.
In most image pickup lenses (optical systems), focusing from an infinitely distant object to the closest object is performed by moving all or part of lens unit(s) in the image pickup lens. The telephoto lens having a long focal length is likely to increase the overall lens size or the overall lens weight, and thus it is structurally difficult to move the entire telephoto lens for focusing.
Thus, the conventional telephoto lens usually moves part of lens units for focusing. Among them, an inner focus type focusing moves part of lens units which is comparatively small and lightweight and located at central rather than front in the optical system. Each of the telephoto lenses in the above three references includes, in order from the object side, a first lens unit having a positive refractive power and a second lens unit having a negative refractive power. The second lens unit is moved to the image plane side for focusing.
In general, the overall lens system becomes larger as a focal length becomes longer in the telephoto lens. It is therefore important for a telephoto lens to maintain a miniaturization of the overall lens system, and to properly and particularly correct the chromatic aberration among a variety of aberrations caused by the increased focal length. Moreover, it becomes important to provide faster focusing a small and lightweight lens unit other than the front lens unit and to lessen the burden of the driving unit.
In general, the front lens unit having a positive refractive power particularly becomes larger and heavier as the focal length becomes longer in a telephoto lens. It is thus important for the telephoto lens to properly set a lens configuration of the front lens unit having a positive refractive power in order to realize the small and lightweight configuration, to properly correct the chromatic aberration, and to obtain a high optical performance. The improper lens configuration of the front lens unit causes the overall system to be larger and amounts of the variety of aberrations to increase, and makes it very difficult to obtain a high optical performance.